RE: definition of liquid hydrogen?

From: Alberto Fasso' <>
Date: Mon, 14 May 2012 10:50:33 -0700 (PDT)


I wrote "That is true for doses of neutrons with energies higher than a few
tens of MeV": of course I meant LOWER than a few tens of MeV. Apologies.

Please keep the mail exchange on the FLUKA discussion list. Other people may
be interested, and other people than me might have a better answer.

I will have a look at your input files as soon as I have some time.


On Mon, 14 May 2012, REDDELL, BRANDON D. (JSC-EV511) wrote:

> Alberto,
> I appreciate your response, but I'm not sure if you are confirming my
> questions or not. Do you agree if my input files (in particular, the set up of
> my liquid hydrogen cards) are correct? I am going to find out how these other
> LH2 dose calculations were validated (if at all!).
> Brandon
> -----Original Message-----
> From: Alberto Fasso' []
> Sent: Monday, May 14, 2012 11:05 AM
> To:
> Subject: RE: definition of liquid hydrogen?
> Dear Brandon,
> you say that "it is well known that liquid hydrogen shielding drastically
> lowers dose". That is true for doses of neutrons with energies higher than a few
> tens of MeV, but not for neutrons of much higher energy.
> For a typical shield of a high energy proton accelerator, the best shielding
> configuration consists of a layer of high-Z (high atomic number) material, such
> as steel, followed by a low-Z shield with high hydrogen content - most often
> concrete. This scheme takes advantage of high inelastic cross sections such as (n,xn) or
> (n,n') in high-Z materials to reduce the neutron energy effectively. The lower
> energy neutrons generated in this process are then best attenuated by moderation
> and absorption down to thermal energies in hydrogenous material.
> Of course, I realize that a steel shield would be difficult to set up in space.
> But don't be surprised that neutrons from 1 GeV spallation are only partially
> shielded by pure hydrogen.
> Alberto
> On Fri, 11 May 2012, REDDELL, BRANDON D. (JSC-EV511) wrote:
>> Thank you for the help with the H2 definition in FLUKA. The problem I am still
>> having is that it is well known that liquid hydrogen shielding drastically
>> lowers dose. In the case that I am running, I am only seeing an ~18% reduction
>> from what aluminum shielding provides. I've attached the two cases. Both cases
>> have approximately the same shielding mass (~55 g/cm2). Whether I run a
>> spectrum of protons (like the GCR protons) or mono-energetic beam of around 1
>> GeV, I get similar results. Other data shows that the reduction should be ~90%
>> or so. Is there something wrong with either my input file or the definition of
>> the material still? If, these input files are correct, then the secondary
>> particles that FLUKA generates contribute 4-5x more than the beam particles
>> (at this shielding thickness)..which suggests the effectiveness of the liquid
>> hydrogen shielding isn't as great as thought to be.
>> Brandon

Alberto Fasso`
SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025
Phone: (1 650) 926 4762   Fax: (1 650) 926 3569
Received on Mon May 14 2012 - 23:44:37 CEST

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